1. Field of the Invention
This invention relates to a process for producing styrene by dehydrogenation of ethylbenzene.
2. Description of the Prior Art
It is known to produce styrene by dehydrogenating ethylbenzene using an iron-based dehydrogenating catalyst, ethylbenzene being diluted with a large amount of steam. The steam used as the diluent has several other functions, e.g., it supplies the heat necessary for dehydrogenation, reduces the partial pressure of the reactants, and decokes the carbon on the catalyst by reaction with an aqueous gas. It is difficult to recover the heat not used in the reaction from the steam and a huge amount of heat is left unrecovered when steam is used in large volume. The reduction of the amount of steam used in the dehydrogenation of ethylbenzene is a greatly desired benefit to process economy and several attempts have been made to achieve this end. A typical example is a reactor of the external heating type. This reactor reduces the amount of steam by supplying the required amount of heat from outside of the reaction pipes, but, because of the use of shell--and--tube pipes to heat the catalyst bed from the outside, a large--scale reactor cannot be installed, and the construction cost is higher than that of the common downflow reactor or radial flow reactor.
Efficient production of styrene by dehydrogenation of ethylbenzene depends on the conversion of ethylbenzene and the selectivity of converted ethylbenzene for styrene, and the higher these parameters, the less steam used. Steam used in the dehydrogenation of ethylbenzene has the following function: (1) it affects the equilibrium conversion of ethylbenzene, i.e. it shifts the equilibrium by reducing the partial pressure of the reactants, and the more steam that is used, the greater the equilibrium conversion of ethylbenzene; (2) it prevents coking during the dehydrogenation; and (3) it supplies the heat required for dehydrogenation. For these reasons, 13 to 16 mols of steam per mol of ethylbenzene are conventionally necessary to produce styrene by dehydrogenation of ethylbenzene, and the conversion of ethylbenzene achieved is only about 60%, and 70% at most. Hence, in the conventional method, a great deal of unreacted ethylbenzene must be recirculated and this results in a corresponding increase in the amount of steam required.
A need therefor continues to exist for a process for the production of styrene by the dehydrogenation of ethylbenzene wherein the energy cost, i.e. the steam requirements, are greatly reduced.